Dental restorations such as crowns and bridges are commonly made by what is known as the porcelain fused to metal process. A metal coping or support structure is covered with layers of glass having different levels of translucency. Opaque layers cover the metal to hide its color followed by more translucent layers to improve the aesthetic appearance. In recent years there has been a trend toward all ceramic dental restorations; crowns, bridges, inlays, etc. In particular, metal copings which provide the structural support for crowns and bridges are being replaced by high strength ceramics. These materials have color and translucency characteristics which better match the natural tooth and produce a more aesthetic appearance.
Zirconia is a preferred material for this application because of its high strength and toughness. Pure zirconia exists in three crystalline forms; monoclinic, tetragonal, and cubic. Monoclinic is stable from room temperature up to about 950-1200° C., tetragonal is the stable form from 1200° C. to about 2370° C., and cubic is stable above 2370° C. Sintering zirconia to high density generally requires temperatures above 1100° C. The monoclinic phase typically transforms to tetragonal during sintering, but then transforms back to monoclinic on cooling. Unfortunately, this transformation is accompanied by a volume expansion which causes the ceramic to crack and usually break apart. Stabilizing agents such as yttria can be added to zirconia to avoid this destructive transformation. Typically, when greater than about 2 mole percent yttria is added, the tetragonal phase can be retained as a metastable phase during cooling. When more than about 8 mole percent yttria is added the cubic phase forms at sintering temperatures and is retained during cooling. Between these levels of yttria a mixture of the tetragonal and cubic phases are formed during sintering and usually retained during cooling. Under rapid cooling conditions the cubic phase may be distorted to form another tetragonal phase known as tetragonal prime.
Zirconia stabilized with 2-3 mole percent of yttria is especially attractive as a structural ceramic because it can exhibit a large degree of transformation toughening. At this level of yttria the material consists largely of metastable tetragonal crystals with the balance being cubic or tetragonal prime. When a crack passes through the material it triggers transformation of the tetragonal crystals near the crack tip to the monoclinic form along with the associated volume expansion. This localized expansion resists the extension of the crack acting as a toughening mechanism.
The amount of toughening is dependent on the grain size, yttria content, and the matrix constraint. As the grain size is reduced the tetragonal form becomes more stable. Optimum toughening is obtained when the grain size is just below the critical grain size where the tetragonal phase is metastable. If the grain size exceeds the critical size the tetragonal phase can convert spontaneously to the monoclinic form throughout the bulk of the material causing widespread cracking. If the grain size is too far below the critical size than the tetragonal crystals are so stable that they will not revert to monoclinic in the stress field of a crack tip. As the amount of yttria stabilizer in the tetragonal form is reduced the tetragonal form becomes thermodynamically less stable and the critical grain size is reduced.
Generally as discussed in Scripta Materialia, 34(5) 809-814 (1996), at an overall composition of 3 mole percent yttria excellent toughening is obtained with grain sizes near 500 nm, but the toughness is reduced at grain sizes near 100 nm. While the overall composition is 3 mole percent yttria, the tetragonal phase contains about 2 mole percent yttria, the remainder is in the cubic phase that is also present. As the bulk yttria content changes from 2-9 mole percent over the range where the tetragonal and cubic phases coexist, the yttria constant of the tetragonal phase itself is constant. As a result the critical grain size is also constant over this range. It should be expected then that as the grain size of the two phase materials is reduced to values approaching 100 nm the effect of transformation toughening will also be reduced. Some insight into the absolute minimum grain size which can provide a toughening effect can be found in studies of pure tetragonal materials where the amount of yttria can be reduced to lower levels. Further as generally discussed in Journal of Acta Materialia, 50, pages 4555-62, (2002), if the amount of yttria is reduced to 1 mole percent, excellent toughening can be obtained at 90 nm, but falls rapidly below about 75 nm.
As the overall yttria composition increases over the tetragonal plus cubic range there will be decreasing amounts of the tetragonal phase present. Therefore, the toughness and strength of materials would be expected to drop as the amount of tetragonal phase in the material is reduced.
Matrix constraint is the resistance adjacent crystals exert on a tetragonal crystal as it tries to transform (expand) against its surroundings. In a fully dense material the adjacent grains provide a high degree of matrix restraint. A porous material provides room for local expansion and therefore less matrix restraint.
In summary, optimum toughening and strength are expected when the grain size is just below the critical grain size for a given yttria content, the material is fully dense, and contains a high fraction of the tetragonal phase. Improvements in optical translucency achieved by grain size reduction must be balanced against the loss in toughness expected at grain sizes below 100 nm, and especially below 75 nm. Improvements in optical transmission which might be expected with higher cubic content must also be balanced by the loss in toughness expected with fewer tetragonal grains.
The high strength and toughness of zirconia makes milling of intricate shapes from fully dense material very difficult. The milling operation is slow and tool wear is high. To overcome this limitation the zirconia may be milled to shape using a partially densified (calcined) body, referred to as a mill block. The mill block is typically 50% dense. It has sufficient strength for handling and is readily milled with minimal tool wear. The shaped restoration can then be heated (sintered) to form a fully dense article which is strong and somewhat translucent. During the sintering process the material shrinks roughly 20% in linear dimensions as it becomes denser. This shrinkage can be accounted for by using optical scanners and computer design to obtain a three-dimensional image of the restoration. This image file can be expanded to compensate for the sintering shrinkage, then transferred to a computer controlled milling machine to produce the restoration. Sintering at high temperature produces the final densified restoration.
While zirconia has a limited amount of translucency, higher translucency is desired to achieve even better appearance for dental applications. Ceramics are often opaque in appearance due to the scattering of light by pores in the ceramic. In order to achieve even a limited level of translucency, the density of the ceramic is typically greater than 99% of theoretical. Higher clarity can require levels above 99.9% or even 99.99%. Two methods known in the art for achieving very high densities in ceramic materials are hot isostatic pressing and spark plasma sintering. However the equipment required for these methods is relatively expensive and is not well suited for use in most dental restoration laboratories. Also, protective atmospheres and/or graphite dies used in this equipment can lead to discoloration of the zirconia by chemical reduction (loss of oxygen from the zirconia).
Another factor which can limit the translucency of ceramics is the presence of two or more solid phases having a different refractive index. In such cases to improve transparency, it is necessary to reduce the size of these phases well below the wavelength of visible light to avoid excessive scattering. Even in single phase materials scattering can occur if the material exhibits birefringence (i.e., has a different refractive index in different crystal directions). Light is then refracted and reflected (scattered) as it crosses grain boundaries from one crystal to another having a different orientation. In this case the crystallite size needs also to be less than the wavelength of visible light to achieve high levels of translucency. For these reasons highly translucent ceramics are often fabricated from single phase, cubic materials which exhibit no birefringence. In the case of zirconia ceramics, however, strength is compromised as the cubic form of zirconia is not transformation toughened.
Sintering of nanoparticles (10-100 nm) is one way to produce small grains in ceramics. The small size increases the driving force for densification (i.e., the reduction in surface area). Unfortunately nanoparticles tend to form strong agglomerates which do not easily break down during pressing operations. The particles within an agglomerate are generally packed more densely than the surrounding particles leading to a non-uniform pore structure in the final sintered body. Obtaining fully dense, highly translucent articles, without the use of high pressure techniques has proven difficult.
Sol-gel processing of nanometer sized particles is one way of avoiding agglomeration and achieving the high density and small grain size desired for both strength and translucency. The difficulty with this processing approach is that it does not lend itself to the production of relatively large articles. It has been successfully applied to the manufacture of fibers, beads, and abrasive grit, but it is not well suited to the production of bulk articles greater than about 1 mm in size. The problem has been in drying the gel where capillary forces lead to high shrinkage and cracking unless relatively lengthy drying techniques are used. In addition, for nanoparticle sols having organic stabilizing agents to keep the particles well dispersed, it can be difficult to remove these organics during heating without crack formation. The dense packing of nanoparticles in the dry bodies means the open pore channels needed to remove volatiles are relatively small leading to pressure build-up within the body. Also, if the organics separate the individual particles of the dry body, shrinkage will occur as the organics are removed. Since organic is most easily volatilized near the surface, non-uniform shrinkage is likely.
It is known that supercritical extraction of liquid from bulk gels can eliminate cracking during drying because there are no capillary forces present. Further, the lack of capillary forces to pull the particles together tends to lead to more relatively open structures commonly referred to as aerogels. Aerogels can have pore volumes of 90% or more. The more open structure of an aerogel would be expected to aid in uniform volatilization of any organics present. However, the low relative density of an aerogel (typically <10% of theoretical) presents a problem as it is generally known that high packing densities of the particles are desirable for densification during sintering. While silica aerogels have been successfully sintered to full density, it has not been considered possible to sinter crystalline aerogels to full density. Silica sinters by a viscous flow process which is much faster than the solid state diffusion mechanisms responsible for sintering crystalline solids.